1. Field of the Invention
The present invention generally relates to miniature microphones used in electronic devices, more particularly to a silicon condenser microphone for converting sound waves to electrical signals.
2. Description of Related Art
A typical condenser microphone generally includes a diaphragm/backplate pair forming a capacitor which varies with sound pressure, and a Field Effect Transistor (FET) circuit for buffering the output signal. Miniature microphones used in electronic devices are typically electret condenser microphones. These are built with highly precision stamped metal parts, organic diaphragm films such as mylar and polyester, and highly charged electret films to bias the microphones. These microphones have certain short-comings. The size has been reduced to the limits of manufacturability. Lack of uniformity in the stamping and assembly processes results in a large variability in sensitivity. Furthermore, temperature and humidity effects on the organic diaphragm film and electret result in long term drift in microphone performance.
In attempts to overcome the difficulties associated with traditional miniature microphones, various workers have tried to make solid state microphones using semi-conductor techniques. The silicon based condenser microphone also known as an acoustic transducer has been in a research and development stage for more than 20 years. Because of its potential advantages in miniaturization, performance, reliability, environmental endurance, low cost, and mass production capability, the silicon microphone is widely recognized as the next generation product to replace the conventional electret condenser microphone that has been widely used in communication, multimedia, consumer electronics, hearing aids, and so on. Of all the silicon based approaches, the capacitive condenser type of microphone has advanced the most significantly in recent years.
The silicon condenser microphone is typically comprised of two basic elements which are a sensing element and a pre-amplifier IC device. The sensing element is basically a variable capacitor constructed with a movable compliant diaphragm, a rigid and fixed perforated backplate, and a dielectric spacer to form an air gap between the diaphragm and backplate. When the diaphragm vibrates, capacitance of the capacitor varies, by which sound waves are converted to electric signals. However, the diaphragm is generally the type of one-piece and the periphery thereof is fixed to the silicon base. When the diaphragm vibrates, a parasitic capacitor will occur between the diaphragm and the silicon base.